1. Field of the Invention
The present invention relates to a method for the measurement of oxygen partial pressures in an atmosphere by means of an electrode-active oxygen sensor and to devices for use in such method.
2. Prior Art
One type of electrode-active oxygen sensor has been disclosed in our copending U.S. patent application Ser. No. 42,229. This type of sensor, which will be referred to hereinafter as a "2-pole, zero .DELTA.E sensor", consists of a metal (1)/solid electrolyte/metal (2) electrolytic cell in which, in its preferred form metal (1) is a palladium foil and metal (2) is a platinum foil.
Such a cell has a quantitative sensitivity towards the partial pressure of oxygen in an atmosphere and the use of the sensor for measuring such partial pressures relies on the detection of a thermodynamic transition temperature (hereinafter referred to as a critical temperature T.sub.c), at which the free energy for the formation of palladium oxide becomes zero on a scale relative to the standard hydrogen electrode. Below T.sub.c, the sensor is in the so-called high resistivity mode (referred to as "hrm") and the cell's resistance is substantially higher than is the resistance at temperatures above the critical temperature, at which temperatures the sensor is in the so-called low resistivity mode (referred to as "lrm").
In a preferred mode of operation, the temperature of the sensor is raised to put it into lrm (about 900.degree. C.). A decreasing linear temperature ramp is then applied and the temperature is noted at which the change lrm - hrm becomes obvious. At this temperature, the resistivity increases excessively compared with the linear rate due to the fall in temperature. This temperature is the critical temperature at which the free energy of formation of the oxide of the active-metal electrode becomes zero and is uniquely related to the oxygen partial pressure of the atmosphere being measured.
The "2 - pole, zero .DELTA.E sensor" operates at zero externally applied potential, and its operation is limited by the temperature condition of the environment. Thus it is not possible to use such a sensor to determine fractional partial pressures in ambient temperatures greater than about 855.degree. C. Furthermore the sensitivity at low oxygen pressures (p.sub.0.sbsb.2 &lt;10.sup.-4 atm) is impeded by the high resistivity of the solid electrolyte at temperatures less than 400.degree. C.